Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today. Such batteries are “secondary” or rechargeable which means they are capable of undergoing multiple charge/discharge cycles. Typically lithium-ion batteries are prepared using one or more lithium electrochemical cells containing electrochemically active materials. Such cells comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material. When a lithium-ion battery is charging, Li+ ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Various electrochemically active materials have been suggested for use as the cathode materials, for example LiCoO2, LiMn2O4 and LiNiO2, see U.S. Pat. No. 5,135,732 and U.S. Pat. No. 4,246,253. However these materials exhibit problems, for example cycle fading (depletion in charge capacity over repeated charge/discharge cycles), which make them commercially unattractive. Attempts to address cycle fading have led to lithium metal phosphate and lithium metal fluorophosphates becoming favourable materials. Such materials were first reported in U.S. Pat. Nos. 6,203,946, 6,387,568, and by Goodenough et al. in “Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries”, Journal of Electrochemical Society, (1997) No. 144, pp1188-1194.
Many workers have tried to provide economical and reproducible synthesis methods for phosphate-containing materials, and especially for high performance (optimised) phosphate-containing materials. A review of the prior art methods which describe the preparation of one particular lithium metal phosphate, namely, lithium iron phosphate (LiFePO4), is given by X. Zhang et al in “Fabrication and Electrochemical Characteristics of LiFePO4 Powders for Lithium-Ion Batteries”, KONA Powder and Particle Journal No. 28 (2010) pp 50-73. As this review demonstrates, a lot of effort has been expended since lithium iron phosphate was first identified in 1997, to find the most expedient method for producing a LiFePO4 material with the best all round performance; for example solid-state synthesis using mechanochemical activation to increase the activation of the starting materials, microwave heating to control the particle size of the active cathode material, and carbothermal reduction which enables Fe(III) e.g. in the form of Fe2O3 or FePO4 (i.e. cheap and readily available sources of iron) to be used as a precursor material. The carbothermal reduction process is a high-temperature reduction reaction (typically 550° C. to 850° C.) which commonly utilizes carbon black, graphite or pyrolyzed organic chemicals as the source of carbon reducing agent. Carbothermal reduction is a highly endothermic reaction, hence the reaction temperature must be sufficient to drive the reaction. In addition, since solid carbon is used as the reducing agent, all the precursors and reactants must be kept in good contact throughout the reaction, nevertheless as reported in the review mentioned above, carbothermal reduction is excellent for the reduction of Fe(III), the stabilization of Fe(II), the control of particle morphology, and the enhancement of electrical conductivity by coating LiFePO4 with residual carbon.
Particulate reducing agents other than carbon, specifically silicon oxide, titanium oxide and elemental metals such as Fe, Co, Ni, Mn, Cu, V, Ti, Cr, Nb, Mo, Mg, Ca, Zn, Sr, Pb, Cd, Sn, Ba, Be, Al and B are disclosed in EP1 343 720.
The most interesting cathode materials are those which have large charge capacity, are capable of good cycling performance, highly stable, and of low toxicity and high purity. To be commercially successful, the cathode materials must also be easily and affordably produced. This long list of requirements is difficult to fulfil but, as detailed in the review mentioned above, the active materials most likely to succeed are those with small particle size and narrow size distribution, with an optimum degree of crystallinity, a high specific surface area and with uniform morphology.
Due to its reactivity elemental phosphorus is never found naturally occurring in the Earth, but calcium phosphate rock is mined and using a carbothermal-type reduction process this is typically converted into phosphoric acid and then ammonium phosphate before being used as a precursor of fertilisers and animal feed. To a lesser extent calcium phosphate rock is also a source of elemental phosphorus which is converted into sodium hypophosphite (NaH2PO2) or phosphorus trichloride which is a precursor of glyphosate (a non-selective herbicide).
Elemental phosphorus exists in a number of forms characterised visually by their colour as white, red, violet and black. The two major forms, white and red phosphorus, behave very differently from each other. The former allotrope contains discrete P4 molecules, is crystalline, highly toxic and highly reactive, e.g. it spontaneously ignites in air at room temperature. Red phosphorus, on the other hand is highly polymeric Pn, amorphous or crystalline, of very low toxicity and of much lower reactivity. Almost half (44%) of the 7000 tons of red phosphorus produced per year is used in the strike strip on match boxes; 24% is used in the manufacture of aluminium phosphide (used in semi conductors and as a fumigant); 18% as flame retardants in plastics, especially for polyamides in electronics, polyurethanes and latex; 6% in pyrotechnics.
In addition to the reactions described above, it is known to use red phosphorus in combination with aqueous hydrogen iodide to provide an effective reducing agent in organic chemistry and the use of red phosphorus in, for example, hydrogenation reactions, deoxygenation of alcohols, ketones, ketoacids and quinones, the cleavage of phenol ethers, and reductive cleavage of lactones are all documented in the literature. Red phosphorus is also known to reduce acidic solutions of metallic salts to yield binary metallic phosphides, as reported by Ludwig Rosenstein in J. Am, Chem. Soc., 1920, 42 (5). pp 883-889.
Further prior art, for example CH515852A, describes the preparation of magnesium phosphide (Mg3P2) using magnesium metal and elemental phosphorus. In this preparation, the oxidation state of the phosphorus changes from P0 (elemental phosphorus), to P3− in Mg3P2; and the oxidation state of magnesium changes from Mg0 (magnesium metal) to Mg2+ in Mg3P2. Thus, this reaction proceeds by the reduction of phosphorus and the oxidation of magnesium.
In prior art document Lin et al. J. Alloys and Compounds 183, 403-412, 1992, phosphides such as Na6WP4, Na5SrNbP4 and Na5SrTaP4 using precursor materials: binary phosphides (Na3P and SrP), red phosphorus and the corresponding metal powder (tungsten, niobium and tantalum). As above, the oxidation state of the red phosphorus metal starting material changes from P0 to P3− in the final products. Thus this reaction proceeds by the reduction of phosphorus and the oxidation of the precursor metal powder materials.
The present invention aims to provide a fast, reliable and cost effective process for the preparation of metal-containing compounds and including but not limited to alkali metal-containing compounds. Advantageously, the process of the present invention aims to provide metal-containing compounds that meet the structural and alkali ion insertion properties needed for commercially viable cathode active materials. To this end, the present invention provides a process for the preparation of a metal-containing compound comprising using i) elemental phosphorus and ii) one or more metal-containing precursor compounds. Preferably, the process of the present invention produces metal-containing compounds that comprise one or more metals which have an average oxidation state that is lower than the average oxidation state of the one or more metals in the one or more metal-containing precursor compounds. Further preferably the process is conducted by heating the reaction mixture to at least 150° C.
Ideally, the present invention is conducted in the absence of an acidic medium that consists essentially of one or more selected from dilute aqueous hydrochloric acid, dilute aqueous sulphuric acid and dilute aqueous nitric acid. Also preferably, the metal-containing compound is other than or in addition to a binary metal phosphide and/or a metal in oxidation state 0. In particular, the present invention is not concerned with the preparation of metal phosphides (compounds consisting essentially of one or more metals and phosphorus) in the absence of any other metal-containing compounds also being prepared, or with the preparation of metals in their elemental state in the absence of any other metal-containing compounds also being prepared, or with the preparation of a combination of such metal phosphides and metals in their elemental state in the absence of any other metal-containing compounds also being prepared. Still further preferably, the process of the present invention employs two or more metal-containing precursor compounds.
In particular, the above process of the present invention provides comprises reacting i) elemental phosphorus with ii) one or more metal-containing precursor compounds wherein the one or more metal-containing precursor compounds comprise one or more elements selected from alkali metals, transition metals, non-transition metals and metalloids. In the context of this invention the term “metalloid” is an element with both metal and non-metal characteristics.
Ideally, the present invention provides a process in which the metal-containing compound comprises one or more metals which have an average oxidation state which is lower than the average oxidation state of the one or more metals in the metal-containing precursor compounds. Further ideally, one or more of the metal-containing compounds and metal-containing precursor compounds comprise one or more transition metals.
In a highly preferred process, the present invention produces compounds containing one or more alkali metals. Such compounds include alkali metal (metal)-containing compounds and are produced by reacting i) elemental phosphorus with ii) one or more metal-containing precursor compounds comprising one or more alkali metals optionally together with one or more metals selected from transition metals and/or non transition metals and/or metalloids. Additional separate transition metal- and/or non transition metal- and/or metalloid-containing precursor compound(s) may also be used, especially, but not exclusively, when the alkali metal-containing precursor compound does not already comprise a transition metal, non transition metal and/or metalloid.
A preferred process of the present invention produces a metal-containing compound of the formula:AaMb(XcYd)eZf wherein:
A is an alkali metal selected from one or more of lithium, sodium and potassium;
M comprises one or more metals selected from transition metals, non-transition metals and metalloids;
(XcYd)e is at least one first anion; and
Z is at least one second anion
wherein a≥0; b>0; c>0; d≥0; e>0 and f≥0;
wherein a, b, c, d, e and f are chosen to maintain electroneutrality.
Desirably, the process of the present invention produces a metal-containing compound, for example of the formula AaMb(XcYd)eZf, in which M comprises one or more transition metals and/or non transition metals and/or metalloids which have an average oxidation state which is lower than the average oxidation state of the one or more metals (transition metals and/or non transition metals and/or metalloids) in the metal-containing precursor compounds.
The most preferred metal-containing compounds produced by the process of the present invention are of the formula:AaMb(XcYd)eZf wherein:
A is an alkali metal selected from one or more of lithium, sodium and potassium;
M comprises one or more transition metals and optionally one or more further metals selected from non-transition metals and metalloids;
(XcYd)e is at least one first anion; and
Z is at least one second anion
wherein a≥0; b>0; c>0; d≥0; e>0 and f≥0;
wherein a, b, c, d, e and f are chosen to maintain electroneutrality.
The addition of elemental phosphorus is crucial to the success of the invention and depending upon the particular one or more metal-containing precursor compounds, and the desired final product, its presence may be either a) as a reducing agent or b) as a source of phosphorus in the final product, or, in many cases, for both of these reasons. When behaving as a reducing agent, at least some of the elemental phosphorus serves to reduce the average oxidation state of the metal components of the metal-containing precursor compounds. This is particularly beneficial in the case where the metal is a transition metal. Preferably at least some of the elemental phosphorus is oxidized during the process of the present invention. When the at least some of the elemental phosphorus is behaving as a source of phosphorus in the metal-containing compound, it is incorporated into the metal-containing compound, for example AaMb(XcYd)eZf, at a higher oxidation state than its elemental state. When elemental phosphorus is used as a source of phosphorus, this may provide a partial phosphorus source in the desired final product, in which case another source of phosphorus (e.g. a phosphate starting material) may also be used, or alternatively, the amount of elemental phosphorus used may be sufficient to obviate the need for any additional phosphate starting material. The fact that the elemental phosphorus is able to act both as the reducing agent and as a source of phosphorus in the metal-containing compound is seen as one of the many advantages of the present invention.
In one group of compounds of the formula AaMb(XcYd)eZf, it is preferable that when a=0, then (XcYd)e is not a phosphide group. Further, the present invention preferably does not include the preparation of one or more binary phosphides (compounds consisting essentially of a metal and phosphorus) in the absence of any other compound of the formula AaMb(XcYd)eZf.
In a particularly preferred process, alkali metal (metal)-containing compounds are prepared. Such compounds may have the formula AaMb(XcYd)eZf where A is one or more alkali metals, M comprises one or more transition metals and/or one or more non transition metals and/or one or more metalloids, and X, Y, and Z are as defined below.
In such alkali metal (metal)-containing compounds, a>0, b>0, c>0, d≥0, e>0 and F≥0.
In the reaction products produced by the process of the present invention:
A preferably comprises one or more alkali metals selected from sodium, lithium and potassium;
M comprises one or more metals selected from transition metals such as titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, aluminium, scandium, yttrium, zirconium, technetium, rhenium, ruthenium, rhodium, iridium, mercury, gallium, indium, tin, lead, bismuth and selenium, non transition metals such as magnesium, calcium, beryllium, strontium and barium, and metalloids such as boron, silicon, germanium, arsenic, antimony and tellurium;
X comprises one or more elements selected from titanium, vanadium, chromium, arsenic, molybdenum, tungsten, niobium, manganese, aluminium, selenium, boron, oxygen, carbon, silicon, phosphorus, nitrogen, sulfur, fluorine, chlorine, bromine and iodine.
Y comprises one or more moieties selected from halides, sulfur, sulfur-containing groups, oxygen, oxygen-containing groups and mixtures thereof;
Z is selected from one or more halides, hydroxide-containing groups and mixtures thereof.
The elemental phosphorus used in the present invention may be any one or a mixture of the polymeric and amorphous allotropes discussed above. Ideally, however, the elemental phosphorus used comprises a major constituent of red phosphorus.
Desirable compounds of the formula AaMb(XcYd)eZf include, but are not limited to, those in which A is lithium and/or sodium, and in which the first anion (XcYd)e comprises one or more groups, preferably selected from phosphate, hypophosphite, condensed polyphosphate, sulfate, oxide, thiosulfate, sulfite, chlorate, bromate, oxyhalide, halide, silicate, arsenate, selenate, molybdate, vanadate groups and any oxyanion groups. Compounds where X comprises phosphorus, for example in which (XcYd)e is a PO4 and/or P2O7 moiety are especially preferred. Similarly, compounds in which X comprises sulfur are equally advantageous, such as those containing SO4 moieties. Compounds of the general formula AaMb(XcOd)eZf are especially preferred.
Other favourable materials include:
LiFePO4,
LiFePO4/Fe2P,
LiMnPO4,
LiCoPO4,
LiNiPO4,
NaFePO4,
NaMnPO4,
NaCoPO4,
NaNiPO4,
LiMn0.5Fe0.2Mg0.3PO4,
Li3V2(PO4)3,
Na4Fe3(PO4)2P2O7,
Na3V2(PO4)3,
LiMn0.5Fe0.5PO4,
Na7V4(P2O7)4PO4,
Na7V3(P2O7)4,
Na2Fe(SO4)2,
NaVPO4F,
LiVPO4F,
Na3V(PO4)2,
Li3V(PO4)2,
NaVOPO4,
LiVOPO4,
LiV2O5,
NaV2O5,
NaVO2,
VPO4,
MoP2O7,
MoOPO4,
Fe3(PO4)2,
Na8−2xFe4+x(P2O7)4,
Na8−2xMn4+x(P2O7)4,
Na2MnP2O7,
Na2FeP2O7,
Na2CoP2O7,
Na4Mn3(PO4)2P2O7,
Na4CO3(PO4)2P2O7,
Na4Ni3(PO4)2P2O7,
NaFeSO4F,
LiFeSO4F,
NaMnSO4F,
LiMnSO4F,
Na2FePO4F,
Na2MnPO4F,
Na2COPO4F,
Na2NiPO4F,
Na2Fe2(SO4)3,
Li2Fe2(SO4)3, and
Li2Fe(SO4)2.
Advantageously, the present invention provides a process for the preparation of phosphate-containing materials comprising using elemental phosphorus as a reducing agent and/or source of phosphorus.
In particular, the present invention provides a process for the preparation of a compound comprising a lithium metal phosphate of the general formula: LiMPO4, where M comprises a metal selected from one or more of manganese, iron, cobalt, copper, zinc, nickel, magnesium and calcium, the process comprising using elemental phosphorus as a reducing agent and/or a source of phosphorus.
Preferred LiMPO4—containing compounds include LiFePO4—containing compounds and these may be produced in the process of the present invention by reacting elemental phosphorus with one or more metal-containing precursor compounds which may include, but are not limited to a selection from LiH2PO4, Li2HPO4, LiOH, LiOH.H2O, Fe3O4, H3PO4, (NH4)2HPO4, (NH4)H2PO4, Fe2O3, Li2CO3, FePO4.xH2O, FePO4, Fe3(PO4)2, FeSO4.xH2O, Fe(NO3)3, Fe(CH3CO2)2, C6H8O7.xFe3+.yNH3 (ammonium iron (III) citrate), C6H5FeO7 (iron (III) citrate). and Fe(C5H7O2)3 (iron (III) 2,4-petanedionate). Any other suitable precursor compound may also be chosen.
A convenient way to perform the process of the present invention is by:                1. forming a mixture comprising i) one or more metal-containing precursor compounds and ii) elemental phosphorus;        2. heating the mixture to a temperature of at least 150° C.; and        3. recovering the resultant product, preferably a metal-containing compound of the formula:AaMb(XcYd)eZf         
wherein A is one or more alkali metals selected from lithium, sodium and potassium, M comprises one or more metals selected from transition metals, non transition metals and metalloids, (XcYd)e is at least one first anion and z is at least one second anion, and wherein a≥0, b>0, c>0, d≥0, e>0 and f≥0.
Further preferably the reaction mixture is heated to a temperature of at least 160° C.
Ideally the starting materials are intimately admixed in particulate form. This can be achieved using various methods, for example by finely grinding the materials separately using a pestle and mortar or a ball mill, and then mixing them together, or the materials can be admixed whilst they are being finely ground. The grinding and admixing is of sufficient duration to produce a uniformly intermixed finely ground powder. If a high energy mixing process is used, it may be advantageous to add the required amount of elemental phosphorus after this step, using a less vigorous mixing process. A solvent such as acetone or another material which is easily removed, for example a low boiling liquid, can be used to assist the grinding/admixing process and this is preferably removed prior to the heating step. Other known techniques such as high energy ball milling and microwave activation may also be used to help prepare the starting materials, for example to increase their reactivity.
The present invention may be performed as a “solid-state” reaction i.e. a reaction in which all of the reactants are in solid form and are substantially free of any reaction medium such as a solvent. It is possible to use a solvent or other low boiling liquid to assist the mixing of the reactants, as described above; preferably when a solid state reaction process is to be used then this solvent/low boiling liquid is substantially removed prior to the heating step.
It is also possible to use a solution based reaction, for example where one or more of the metal-containing precursor compounds is mixed with or dissolved in a solvent such as water, and in addition, or alternatively, one or more other precursor compounds, such as P2O5 and H3PO4 may be conveniently mixed with or dissolved in water.
The reaction between the starting materials (where some or all are either in solid form, or mixed and/or dissolved in a solvent) generally occurs during the heating step of the process, which typically involves heating the reaction mixture either at a single temperature, or over a range of temperatures, for example greater than 150° C., preferably up to at least 160° C., preferably at least 200° C. and further preferably up to at least 300° C. A single or a range of reaction temperatures of from greater than 150° C. to 1200° C. is preferred, with from at least 200° C. to 1200° C. being very preferred and 150° C. to 800° C. being particularly preferred. Such a heating regime is quite different from that described in the prior art which boils red phosphorus and metallic salts in dilute aqueous acidic solutions; the latter would utilise a considerably lower reaction temperature of close to 100° C.
Conveniently the reaction is performed under atmospheric pressure and under a non-oxidising atmosphere, for example nitrogen, argon or another inert gas, or under vacuum. If using an inert gas during the reaction, it is beneficial to flush the reaction vessel with the inert gas in order to expel any air present. In some cases it is also advantageous to use a low flow rate of said gas, for example less than 4 litres/minute.
Advantageously, and highly unexpectedly it has also been found that the reaction process of the present invention may also be performed under an atmosphere comprising a partial pressure of oxygen. In this case, the oxygen may be obtained or derived from any convenient source, for example oxygen gas, such as sourced from an oxygen gas cylinder or other suitable container, or an oxygen precursor material such as hydrogen peroxide (e.g. by heating or in the presence of a catalyst), other peroxides, metal nitrates, potassium permanganate, potassium bromate or water (e.g. by electrolysis), or obtaining oxygen from an oxygen generating biomaterial.
Therefore, the process of the present invention comprises the steps of:                1. forming a mixture comprising i) one or more metal-containing precursor compounds and ii) elemental phosphorus;        2. heating the mixture to a temperature of at least 150° C.; and        3. recovering the resultant product, preferably a metal-containing compound of the formula:AaMb(XcYd)eZf wherein A is one or more alkali metals selected from lithium, sodium and potassium, M comprises one or more metals selected from transition metals, non transition metals and metalloids, (XcYd)e is at least one first anion and z is at least one second anion, and wherein a≥0, b>0, c>0, d≥0, e>0 and f≥0; wherein step 2. is optionally carried out under an atmosphere comprising a partial pressure of oxygen.        
Preferably the atmosphere comprising a partial pressure of oxygen may also comprise other gases such as nitrogen and/or one or more inert gases such as argon. Air is a particularly useful and cost effective source of oxygen. The exact partial pressure of oxygen needed for the reaction of the present invention is preferably calculated to be sufficient to ensure a stoichiometric amount of oxygen; an excess of oxygen might cause unwanted side reactions with the elemental phosphorus, and too little oxygen could result in a reduction in the yield of the target product.
The reaction scheme using a partial pressure of oxygen for one favoured target material is as follows:0.5Li2CO3+0.5Fe2O3+1P+1O2→LiFePO4+0.5 CO2 
Depending on the target material and the precursors used, the process of the present invention may be performed in a sealed reaction vessel. A preferred sealed reaction vessel is a Carbolite tube furnace, comprising a non-porous ceramic tube of 75 mm internal diameter.
A sealed reaction vessel may also be used when conducting the reaction under a partial pressure of oxygen; however, in such a case it is desirable to maintain a substantially constant partial pressure of oxygen. This may be achieved by accommodating the volume changes which naturally occur as the reaction temperature changes during the process, for example during the heating step. The provision of inflatable expansion vessels attached to the reaction vessel, such as described in detail below, has been found to be a particularly convenient way to achieve this. It is also highly convenient to tailor the amount of precursor reactant mix to the volume of oxygen (for example, derived from air) contained within a sealed reaction vessel.
Advantageously, the reaction temperature is maintained for between 0.5 and 72 hours, although the exact time will depend on the reactivity of the starting materials. Between 0.5 and 8 hours has been found to be sufficient for many reactions utilising the process of the present invention, particularly when the reaction temperature is 500° C. or above. When the reaction synthesis is performed at relatively low temperatures, for example 400 ° C. or below, then using a longer dwell time of 12 to 72 hours, preferably 24 to 48 hours, is found to achieve the desired reaction product. The exact reaction temperature and dwell time will be chosen to provide the most commercially advantageous reaction process for a given target material.
As discussed above, elemental phosphorus has two potential roles in the process of the present invention; a) as a metal (transition metal and/or non transition metal and/or metalloid) reducing agent, and b) as a source of phosphorus. Which role it plays will depend on the particular reactants, the reaction atmosphere being used, and the quantities of reactants used. In the case where the elemental phosphorus behaves only as a reducing agent, the amount of elemental phosphorus required will depend on the number of electrons being gained by the metal M during the reaction process. For example, in reactions using iron as the metal M, the expected phosphorus redox scheme for iron reduction (Fe3+→Fe2+) is:P0→P5++5e−e−+Fe3+→Fe2+
Giving an overall molar ratio of:5Fe3++P0→5Fe2++P5+In other words 0.2 moles of phosphorus should be required to reduce 1 mole of Fe2+.
Similarly, in reactions using vanadium as the metal M, the expected phosphorus redox scheme for vanadium reduction (V5+→V3+) is:P0→P5++5e−2e−+V5+→V3+
Giving an overall molar ratio of:5V5++2P0→5V3++2P5+
In this case, 0.4 moles of phosphorus should be needed to reduce 1 mole of V5+.
In a preferred reaction scheme, lithium iron phosphate is prepared according to the process of the present invention from: a lithium-containing precursor material (for example lithium carbonate), a source of phosphorus (for example lithium dihydrogen phosphate), a source of iron (III) (for example iron (III) oxide and/or iron phosphate (FePO4)) and elemental phosphorus.
Based on the reaction schemes above, if Fe2O3 is the source of iron, the following molar ratio can be deduced:0.1Li2CO3+0.8LiH2PO4+0.5Fe2O3+0.2P→1LiFePO4+0.1CO2+0.8H2O
In another preferred reaction scheme, a phosphorus source such as lithium dihydrogen phosphate may be omitted and the elemental phosphorus may be used as the sole source of phosphorus in the target metal-containing compound. Alternatively, lithium dihydrogen phosphate may be replaced with one or more other phosphate-containing materials or further alternatively replaced by one or more non-phosphate-containing materials such as phosphorus pentoxide (P2O5) as the source of phosphorus. Some or all, of the source of phosphorus, may be provided by one or more of any of elemental phosphorus, phosphate- and non-phosphate-containing materials.
The metal-containing materials of formula AaMb(XcYd)eZf prepared by the process of the present invention are suitable for use in many different applications, for example as the active material in electrodes, particularly cathodes used in energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. This is especially the case for alkali metal (metal)-containing materials. Advantageously, the electrodes made using the materials produced by the present invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).
An inherent problem with a number of metal-containing compounds, especially alkali metal-containing compounds, is their low electrical conductivity. To address this problem it is known to add conductive materials such as carbon-containing materials for example, graphite, carbon black, sucrose and acetylene black either to the starting materials, such as during grinding, or as a coating to the final metal-containing products. Other known conductive materials include metal powders and other highly conductive inorganic materials.
It is therefore desirable, when making inherently non-conductive materials such as alkali metal-containing compounds using the process of the present invention, to add one or more conductive materials to the reaction mixture and/or to one or more of the starting materials and/or to the final product.
Notwithstanding the above, another particularly useful advantage of the process of the present invention which uses elemental phosphorus, is that the reaction surprisingly produces AaMb(XcYd)eZf compounds, and indeed non-conductive compounds such as alkali-metal-containing compounds, which exhibit significantly better electrochemical results than would be expected for similar compounds made using other methods that do not employ elemental phosphorus. As discussed above, it is usually the case that to obtain a high specific capacity material with low voltage polarization, that is voltage hysteresis between the charge and discharge processes, it is necessary for the electrode material to include intimately dispersed conductive material such as carbon, either during the synthesis step (for example by a carbothermal process) or by the use of a secondary carbon coating process. However, the Applicant has found that there is no need to add a separate conductive material to the compounds produced by the process of the present invention. The reason for this is that they exhibit excellent performance without the addition of carbon to the reaction mixture. Furthermore, the Applicant has also observed excellent electrochemical results even when carbon is not added during the formulation of the electrode.
It is believed that the AaMb(XcYd)eZf compounds produced by the process of the present invention may, where the starting materials so favour, be in the form of a composite material that includes a conductive compound which is preferably produced in situ during the reaction between the metal-containing precursor compounds and elemental phosphorus. The conductive compound formed in situ is preferably a phosphorus-containing compound, and a suitable conductive material may comprise, at least in part, a transition metal phosphide- and/or a non-transition metal phosphide- and/or a metalloid phosphide-containing material such as, in the case where the metal component M comprises iron, an iron phosphide-containing material, for example Fe2P. This latter iron phosphide material in particular, is known to be highly conductive.
Thus in a second aspect, the present invention provides a composition comprising a metal-containing compound e.g. of the formula AaMb(XcYd)eZf as defined above, and one or more conductive materials, wherein at least a portion of the one or more conductive materials is formed in situ during the process comprising reacting i) elemental phosphorus with ii) one or more metal-containing precursor compounds. Desirably the invention provides a composition comprising LiFePO4 and at least one conductive material comprising one or more phosphide-containing compounds. Suitable phosphide-containing compounds may include, but are not limited to binary phosphides.
Further, in a third aspect, the present invention provides a process for preparing a composition comprising a metal-containing compound, e.g. of the formula AaMb(XcYd)eZf defined as above, and one or more phosphorus-containing conductive materials, comprising the step of reacting i) elemental phosphorus with ii) one or more metal-containing precursor compounds.
In a fourth aspect, the present invention provides an electrode which utilises active materials of formula AaMb(XcYd)eZf, prepared in accordance with the present invention as described above, especially an electrode which utilises a composition comprising such active materials in combination with a phosphorus-containing conductive material, and particularly a phosphorus-containing conductive material which has been made, at least in part, during the reaction process described above involving elemental phosphorus.
In still further aspects, the present invention provides an energy storage device comprising an electrode as described above, for use as one or more of the following: a sodium ion and/or lithium ion and/or potassium ion cell; a sodium metal and/ or lithium metal and/or potassium metal ion cell; a non-aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell; and an aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell. Specifically, the energy storage device may be a battery.